When a patient is recovering, it is important to `wean` him from complete dependence on the ventilator. To effect this, ideally the ventilator should be capable of being operated in a mode in which spontaneous breathing (or ventilation) of a patient is both enabled and encouraged, while his ventilation performance is continually monitored against chosen standards so that the ventilator will revert automatically into its automatic mode should the patient's ventilation deteriorate to an unacceptable extent.
Methods used hitherto do not satisfy this requirement, as the monitoring function has to be carried out concurrently with manual adjustments to the ventilator's controls. Methods have been used in the past whereby the patient's end-tidal carbon dioxide level is monitored and made to adjust the inspired tidal volume automatically but these methods are both complicated and very expensive. Furthermore they have not so far been developed to the point of reliability or of satisfying the clinical requirements for weaning patients.
The present invention aims at providing a lung ventilator in which the minute volume of the exhaled gases is continuously monitored so that whenever a patient ceases to exhale sufficient gas per minute the ventilator automatically takes over from the patient.